I think it is actually only very true in a small number of cases. First, you need to be big enough where the cost of the people needed to support your infrastructure can be amortized over a (very) large number of machines. Second, Amazon, Microsoft and Google pay some of the best salaries around, so they have some of the best infrastructure people working for them.

Looking at the comments here, I think it's clear that there are a relatively small number of use cases where roll your own is a better idea, primarily where you have a huge number of servers basically all doing the same thing with lots of data transfer (which is comparatively expensive in the cloud). This may be cheaper to manage when a small team of people if you're essentially cloning similar setups.